1. Techincal Field
The present invention relates to an air-fuel ratio and engine control system for internal combustion engines. More particularly, the present invention relates to the control of the air-fuel ratio and other engine parameters in response to a ratio of cylinder pressures as a function of rotational crankshaft angles.
2. Background Art
Currently, various methods of controlling the combustion process in internal combustion engines are known. Adjustments to controlling the energy conversion function of an engine during combustion are obtained by sensing at least one engine operating condition, such as coolant temperature, manifold pressure, engine speed, mass airflow into the engine, throttle angle, fuel temperature, fuel pressure, fuel rate, EGR rate, exhaust emissions, etc., and adjusting the energy conversion in response thereto. Usually, engine control is determined by varying certain engine operating conditions on a control reference engine to determine the proper energy conversion for the various operating conditions. The problem encountered with this approach is that the engine being controlled is not necessarily the same as the control test engine used for reference, due to manufacturing differences and aging. Therefore, the operating condition being sensed can provide an inaccurate control variable for engine control. In order to overcome this problem, a control system must be implemented with the capability to adjust for these differences and changes. Such a control system is possible using combustion chamber pressure sensors and applying feedback control to ignition timing, EGR rate, or fuel rate.
In a typical engine control, the three controlled combustion parameters are spark timing, EGR rate, and air-fuel ratio. The first parameter affects the timing of the initiation of the combustion process and the latter two affect the speed and duration of the combustion process, while all three parameters affect engine emissions. Air-fuel ratio is generally controlled in a closed loop by an exhaust oxygen sensor to produce a constant stoichiometric ratio for emission control by oxidizing and reducing catalysts in the exhaust system. Since the efficiency of one or the other catalyst falls rapidly as the air-fuel ratio strays even slightly from stoichiometric in either direction, this parameter must be strictly controlled and is not available for maximizing power or fuel efficiency. Internal combustion engines in most cars today typically operate stoichiometrically. Stoichiometric conditions exist when there is exactly the right amount of oxygen available to convert all of the fuel molecules to CO.sub.2 and H.sub.2 O. Under these conditions, there is very little, if any, oxygen in the exhaust to prevent the oxygen from interfering with the catalytic removal of NO.sub.X emissions. Furthermore, there is also virtually no unburned fuel or CO in the exhaust.
However, it has been found that there are situations when it is advantageous to operate with a very lean air-fuel ratio rather than a stoichiometric air-fuel ratio, such as to produce better fuel economy or reduce exhaust emissions. Lean mixtures provide numerous additional advantages as well, such as lowering combustion temperatures which lowers NO.sub.X emissions, increasing efficiency through a higher ratio of specific heats, lowering exhaust temperatures which increases durability, especially at high loads, and having a greater knock margin which allows higher compression ratios to be used resulting in better efficiency. When operating with a very lean air-fuel ratio, existing exhaust gas oxygen sensors cannot accurately measure the exhaust oxygen concentration, which results in inaccurate control of the air-fuel ratio. Therefore, it is desirable to provide an engine control system that easily and reliably is able to control engine operation at lean air-fuel ratios.
As previously stated, combustion chamber pressure sensors can be utilized along with applying feedback control to provide control of engine operation. One such system is disclosed in U.S. Pat. No. 4,996,960 issued to Nishiyama et al., which teaches an air-fuel ratio control system for an internal combustion engine using a ratio of two cylinder pressure measurements, one at top dead center (TDC) and one at 60.degree. before TDC (BTDC), in conjunction with the intake air temperature to calculate a correction for the delivered fuel flow during acceleration or deceleration and thus changing the air-fuel ratio. This control system uses the well known polytropic behavior of the air-fuel mixture that is typically observed during the compression stroke in the cylinder to estimate the charging efficiency and, once the charging efficiency is known, to correct for changes in air flow without the use of an air flow meter. Nishiyama et al. teach taking all cylinder pressure measurements at or before TDC, which is prior to combustion, and their control system does not measure any parameters during the actual combustion event. Therefore, this air-fuel ratio control system would not be able to accurately control the air-fuel ratio of a lean burn engine, which requires the quality of combustion to be monitored.
U.S. Pat. No. 4,622,939 issued to Matekunas discloses a method of controlling spark timing for achieving the best torque in an internal combustion engine by comparing the ratio of combustion chamber pressure to motored pressure for several predetermined crankshaft rotational angles, namely at least 10.degree. and 90.degree. ATDC. The motored pressure is a calculated value of the estimated pressure at 10.degree. and 90.degree. ATDC based upon initial pressure measurements taken at 90.degree. and 60.degree. BTDC, and a ratio between the first and second ratios of combustion chamber pressure to motored pressure at 10.degree. and 90.degree. ATDC is calculated to adjust the ignition timing to maintain a predetermined ratio between the first and second pressure ratios for MBT. Therefore, this control system requires numerous calculations and additional sampling of the pressure signal to determine the motored pressures and all of the ratios as well as additional memory to store all of these calculations. Additionally, the pressure ratio calculated at 90.degree. ATDC occurs at substantially complete combustion, wherein pressure measurements taken late in the combustion cycle are particularly sensitive to measurement errors, such as thermal shock. Thermal shock occurs as the transducer is exposed to hot and cold gases and its body deforms due to thermal expansion of the transducer body, which, in turn, moves the transducer's diaphragm and causes an error which is nearly impossible to remove. Therefore, measurements at substantially complete combustion as implemented by Matekunas are likely to have too great an error to allow adequate precision in the measured pressure ratio. Further, the purpose of the Matekunas invention is to adjust the spark timing to keep the 50% point of combustion relatively fixed in order to achieve MBT timing, and the Matekunas invention does not control the air-fuel ratio. Accordingly, there is a need for an engine control system which is not affected by thermal shock and which does not require a plurality of pressure samplings and a large amount of memory to store calculations of such pressure samplings. There is further a need for an engine control system which adequately functions with a lean air-fuel ratio.
One approach to controlling the operation of an internal combustion engine at lean air-fuel ratios is disclosed in U.S. Pat. No. 4,736,724 issued to Hamburg et al. This control system uses an in-cylinder pressure sensor and a sensor for monitoring the airflow into the engine in a combustion pressure feedback loop, wherein the sensors are attached to a compensation device coupled to the fuel controller. The compensation device modifies the fuel air command applied to the engine as a function of airflow and in-cylinder pressure. The engine's air-fuel ratio is maintained at the lean limit based on continuously measured in-cylinder combustion pressure signals. This control system performs a constant heat release calculation to measure the burn duration, and requires a fast time response in the feedback loop as the burn duration is compared with the lean limited preprogrammed in a burn duration table. Therefore, this control system requires a great deal of processing power and storage memory to continuously monitor the in-cylinder pressure to calculate burn duration. Furthermore, this control system requires the additional measurement of the airflow into the engine which further complicates the required components of the control system and adds another variable to the calculations, which increases the opportunity for error.
Accordingly, there is clearly a need for an engine control system which provides for effective control of the air-fuel ratio at lean conditions while not requiring a plurality of complex calculations and a large amount of memory to store such calculations. Further, there is a need for an engine control system which adequately controls an internal combustion engine at a lean air-fuel ratio in a simpler and more efficient manner.